1. Field of the Invention
This application relates broadly to automatic backwash, seriated beds filter systems. More particularly, it is concerned with a new type of sliding shoe unit used to seriately backwash the multiple filter beds in such filter systems.
2. Description of the Prior Art
Automatic backwash filter systems that include a filter tank divided into a multiplicity of filter beds of substantially the same width by a plurality of vertical cell sheets are a know type of equipment used in water treatment plants. Such automatic backwash filter systems (ABWS) are exemplified in U.S. Pat. Nos. 3,239,061; 4,133,766; 4,617,131; 4,764,288 and 5,089,130 currently assigned to Infilco Degremont Inc., the assignee of the invention disclosed herein. The disclosures of these patents are incorporated herein by reference.
Typically the ABWS include a wear strip through which extend base ports for each separate cell and a sliding shoe which moves along the wear strip to seriately address the cell ports in the progressive backwashing of the filter cells. In conventional ABWS, the sliding shoe has three ports, i.e., one backwash port in the center and two filter to waste ports, i.e., one forward and one behind the backwash port. In some systems, the sliding shoe may comprise additional ports on one or both sides of the single backwash port.
In conducting the backwash of filter cells, a carriage moves the sliding shoe from a first backwashed cell to an adjacent second cell so it can be backwashed. This second cell is then washed with wash water entering the shoe's center backwash port. At the same time, the shoe's trailing filter to waste port is positioned at the effluent port of the first cell, backwashed just previous to the one currently being backwashed.
This first cell is now filtering, but the water being discharged from the cell's effluent port is the remainder of the wash water that was in the media bed at the end of the wash. Since it may contain some suspended matter and not meet the requirements of an acceptable filter effluent, it is directed to waste. Typically, the filters are constructed so this sliding shoe is positioned in the filter effluent channel. Therefore, it is important to maintain a positive seal between the filter to waste port in the sliding shoe and the wear strip to prevent water exiting from the first cell that may contain some suspended matter from entering the effluent channel and combining with the properly filtered water. The necessary seal is provided by holding the shoe tightly against the wear strip via spring or other applied pressures. The present invention provides an apparatus for mitigation of any leakage of water containing suspended solids into the effluent channel.
When sequentially backwashing a plurality of seriate filter beds, it is advantageous to end the backwash cycle of any given media bed not only on a time basis as is usually employed, but in addition, determining when suspended solids are no longer exiting the filter and adequately filtered water is being produced. In this method the carriage moves to backwash the next serial filter cell only after it has been determined that the filter to waste water from the first filter cell no longer contains suspended matter and the time allotted for the backwash of the second filter cell has expired. A turbidimeter or a particle counter can be used to determine when suspended solids have been reduced to the desired level (see U.S. Pat. No. 5,089,117).
In such monitoring procedures, validity of the instrument responses can be questionable if cross contamination occurs because of poorly sealed ports between the sliding shoe used to direct the flow of filter backwash water to a second filter cell and the filter to waste stream of a first filter and the wear strip. The present invention provides an apparatus for mitigation of potential cross contamination.